1. Field of the Invention
The present invention relates to a deflection device for a projection tube, and a projection tube apparatus provided with the deflection device.
2. Description of the Related Art
Recently, in order to satisfy market demands for the enlargement of a screen and the reduction in price, a projection TV is being used widely. FIG. 1 is a schematic view showing a configuration of the projection TV. The projection TV includes projection tube apparatuses 1 to 3 corresponding to respective colors of red, green, and blue, and the projection tube apparatuses 1 to 3 respectively are provided with a deflection device 4. Rasters generated from the projection tube apparatuses 1 to 3 of the respective colors are projected onto a screen 5, whereby a color image is formed.
FIG. 2 is a side view of the deflection device 4 provided on each of the projection tube apparatuses 1 to 3 used for the projection TV. A main deflection device 6 generating a raster is composed of a main horizontal coil 43, a main vertical coil 7, and a main core 8. On a neck side of the main deflection device 6, there is provided an auxiliary deflection device 9 for correcting a raster shift (misconvergence) occurring on the screen 5 of the projection TV due to the error when the projection tube apparatuses (projection tube apparatuses 1 to 3 in FIG. 1) of the respective colors of red, green, and blue are incorporated into a projection TV set.
FIG. 3 is a front view of the auxiliary deflection device 9. Herein, an XYZ rectangular coordinate system is defined, in which a tube axis of the projection tube apparatus (projection tube apparatuses 1 to 3 in FIG. 1) having the main deflection device 6 is a Z-axis, a horizontal axis orthogonal to the Z-axis is an X-axis, and a vertical axis orthogonal to the X-axis and the Z-axis is a Y-axis. The auxiliary deflection device 9 is composed of an auxiliary core 10, an auxiliary horizontal coil 11, and an auxiliary vertical coil 12. The auxiliary horizontal coil 11 is wound in a toroidal shape in the vicinity of a position of the auxiliary core 10 where the X-axis crosses, and the auxiliary vertical coil 12 is wound in a toroidal shape in the vicinity of a position of the auxiliary core 10 where the Y-axis crosses. An appropriate current is supplied to the auxiliary deflection device 9 with such a configuration, whereby a convergence is corrected so as to obtain an appropriate color image without any color displacement on the screen (screen 5 in FIG. 1) of the projection TV.
In the case of a toroidal auxiliary deflection coil, as shown in FIG. 4A, for example, a deflection magnetic field 13 of the main horizontal coil 43 of the main deflection device 6 is interlinked with the auxiliary horizontal coil 11, whereby an induced voltage Vt in a pulse shape with the same period as a horizontal period is generated between terminals of the auxiliary horizontal coil 11, as shown in FIG. 4B. Thus, in order to correct a convergence appropriately, a high voltage for canceling the induced voltage Vt is required as a driving voltage for the auxiliary deflection device, which increases power consumption.
Furthermore, as shown in FIG. 5, the auxiliary horizontal coil 11 and the auxiliary vertical coil 12 are wound in a toroidal shape. Therefore, a magnetic field generated from a winding present outside with respect to the auxiliary core 10 becomes a leakage magnetic field 14 that does not contribute to the deflection, resulting in a low deflection efficiency.
JP 2002-330446 A discloses a deflection device that solves the above-mentioned problems. FIG. 6A is a front view of an auxiliary deflection device 19 of JP 2002-330446 A, and FIG. 6B is a side view thereof. The auxiliary deflection device 19 is composed of a saddle-type auxiliary horizontal coil 15, a saddle-type auxiliary vertical coil 16, and an auxiliary core 17. The auxiliary horizontal coil 15 and the auxiliary vertical coil 16 respectively generate an auxiliary horizontal magnetic field 18 and an auxiliary vertical magnetic field 20. The auxiliary horizontal coil 15 is of a saddle type. Therefore, for example, the number of magnetic fluxes of the deflection magnetic field 13 of the main horizontal coil 43 of the main deflection device 6, which are interlinked with the auxiliary horizontal coil 15, is small. Thus, an induced voltage Vs generated between terminals of the auxiliary horizontal coil 15 is smaller than the induced voltage Vt, so that the power consumption can be reduced.
FIG. 7 is a cross-sectional view taken along a line VII—VII in FIG. 6B. The auxiliary deflection device 19 is symmetrical with respect to an XZ-plane and a YZ-plane. Therefore, FIG. 7 shows only a first quadrant. Both of the auxiliary horizontal coil 15 and the auxiliary vertical coil 16 are of a saddle type, and their windings are wound inside of the auxiliary core 17, so that the deflection efficiency thereof is higher than that of the toroidal type.
In the above-mentioned deflection device of JP 2002-330446 A in which both of the auxiliary horizontal coil 15 and the auxiliary vertical coil 16 are of a saddle type, although the power consumption can be reduced by decreasing an induced voltage caused by a main deflection magnetic field generated by the main deflection coil, compared with the deflection device in which both of the auxiliary horizontal coil 11 and the auxiliary vertical coil 12 are of a toroidal type, there is a problem that the effect of the reduction in power consumption is not sufficient. Hereinafter, the reason for this will be described with reference to FIGS. 7 and 8.
FIG. 8 is a graph showing results obtained by measuring a Y-axis direction component By of the auxiliary horizontal magnetic field 18 generated by the auxiliary horizontal coil 15 along the X-axis. As shown in FIG. 8, the Y-axis direction component By becomes maximum when X=0, and decreases with distance from X=0 (i.e., Y-axis). A magnetic field 21 having such an intensity distribution generally is called a barrel type. As shown in FIG. 7, when a winding angle θH of the auxiliary horizontal coil 15 is larger than 30° and smaller than 90°, the auxiliary horizontal magnetic field becomes a barrel-type magnetic field. Herein, the winding angle θH is defined by an angle, in an XY-plane, formed by the X-axis and a straight line passing through the Z-axis and a midpoint of the auxiliary horizontal coil 15 in a circumferential direction with respect to the Z-axis, which is present in each quadrant partitioned by the XZ-plane and the YZ-plane. The winding angle θH at which the deflection efficiency of the auxiliary horizontal coil 15 becomes maximum is 0°. As the winding angle θH increases, the deflection efficiency decreases, and becomes minimum when θH=90°.
The auxiliary horizontal coil 15 has been described above. The above description also applies to the auxiliary vertical coil 16. A winding angle θV of the auxiliary vertical coil 16 is defined by an angle, in the XY-plane, formed by the Y-axis and a straight line passing through the Z-axis and a midpoint of the auxiliary vertical coil 16 in a circumferential direction with respect to the Z-axis, which is present in each quadrant partitioned by the XZ-plane and the YZ-plane.
As is understood from FIG. 7, in the deflection device of JP 2002-330446 A, respective inner diameters DH, DV of the auxiliary horizontal coil 15 and the auxiliary vertical coil 16 are the same, and in order to prevent the auxiliary horizontal coil 15 and the auxiliary vertical coil 16 from interfering with each other, the respective winding angles θH, θV cannot help being set to be larger than 30°. Consequently, the magnetic fields generated by the auxiliary horizontal coil 15 and the auxiliary vertical coil 16 both become barrel-type magnetic fields. Thus, in the deflection device of JP 2002-330446 A, the deflection efficiency is low, and although the induced voltage Vs caused by the deflection magnetic field 13 of the main horizontal coil 43 can be reduced by making the auxiliary horizontal coil 15 and the auxiliary vertical coil 16 to be a saddle type, the power consumption cannot be reduced sufficiently as a whole.
Furthermore, as shown in FIG. 9, by making the inner diameter DH of the saddle-type auxiliary horizontal coil 15 to be different from the inner diameter DV of the saddle-type auxiliary vertical coil 16, the winding angles θH, θV can be set appropriately to a certain degree. However, in this case, it is necessary to stack the auxiliary horizontal coil 15, the auxiliary vertical coil 16, and the auxiliary core 17 successively from the Z-axis toward the outside, which increases the inner diameters of the auxiliary vertical coil 16 and the auxiliary core 17, and enlarges the distance of the auxiliary vertical coil 16 and the auxiliary core 17 from an electron beam, resulting in a decrease in a deflection efficiency.
Furthermore, generally, in the case of a projection TV, the auxiliary deflection device has a function of correcting pincushion distortion in upper and lower portions of a screen, as well as the function of correcting a convergence. The power required for correcting pincushion distortion in upper and lower portions of a screen is remarkably larger than the power consumption during correction of a convergence. Thus, in order to efficiently reduce the power consumption with a projection TV set, it is important to enhance the deflection efficiency of the auxiliary vertical coil for deflection, particularly in vertical directions of the auxiliary deflection device.
Furthermore, in the deflection device of JP 2002-330446 A, both of the auxiliary horizontal magnetic field and the auxiliary vertical magnetic field are barrel-type magnetic fields, as described above. Therefore, the spot shape of an electron beam in a circumferential region of a screen changes when the auxiliary deflection device is operated. FIG. 10 shows beam spots in a first quadrant of a screen. First, an electron beam 22 on the X-axis will be described. The auxiliary horizontal magnetic field is of a barrel type, so that a Lorentz force Fa acting on an electron 22a of the electron beam 22 closer to the Y-axis is larger than a Lorentz force Fb acting on an electron 22b of the electron beam 22 farther from the Y-axis. Consequently, the electron beam 22 has a vertically oriented shape as represented by a broken line 23, which degrades image quality. An electron beam 22′ on the Y-axis has a horizontally oriented shape as represented by a broken line 23′ due to the barrel-type auxiliary vertical magnetic field for the same reason as the above. Therefore, the cross-sectional shape of an electron beam is varied depending upon the degree of an operation of the auxiliary deflection device, and there is a great variation among mass-produced deflection devices.